Metal-oxygen batteries, which are also referred to as metal-air batteries, are a class of electrochemical cells in which oxygen, which is typically obtained from the ambient environment, is reduced at a catalytic cathode surface as part of the electrochemical cell reaction. Reduction of the oxygen forms an oxide or peroxide ion which reacts with a cationic metal species. Metal-oxygen batteries have been developed based upon Fe, Zn, Al, Mg, Ca, and Li. It is acknowledged that in the strictest sense a “battery” is an electrochemical device comprised of a number of separate electrochemical cells interconnected to a single set of terminals to provide an output which is cumulative in terms of voltage and/or current of each of the individual cells. However, for purposes of the present disclosure, and in keeping with vernacular terminology, the term “battery” will be used herein to define electrochemical power generation and/or storage devices comprised of single cells as well as plural, interconnected cells.
Lithium-oxygen batteries represent one type of metal-oxygen battery. In devices of this type, an electro-active cathode and a lithium-containing anode are disposed in an electrolyte which provides for ionic communication therebetween. During the discharge of the cell, oxygen is reduced at the electro-active cathode to produce O−2 and/or O2−2 ions which react with the lithium to produce Li2O2 and/or Li2O which deposits on the cathode. Such cells provide an operating voltage in the typical range of 2.0-2.8 V, and an open circuit voltage of 2.85 V, and they have a good charge storage capacity.
The electrolyte is an important component of any battery, since it functions to solvate, and provide transport for, the electro-active components of the battery system. Typical electrolytes comprise a solvent having one or more electrolyte salts dissolved therein. Heretofore, conventional wisdom with regard to metal-oxygen batteries has held that solvents for the electrolytes of such batteries should have a very high dielectric constant so that they can effectively solvate electrolyte salts so as to provide for high ionic conductivity, and that they have high boiling points. U.S. Pat. No. 5,510,209 discloses a lithium-oxygen battery, and specifically teaches that the electrolyte component of this battery includes solvents which meet the foregoing criteria.
In accord with the present invention, it has been found that the solubility of oxygen in a solvent is the primary factor which must be considered when formulating electrolytes for metal-oxygen batteries. As will be explained hereinbelow, solvent systems prepared in accord with the present invention provide metal-oxygen batteries which manifest performance characteristics such as discharge capacity and rate capability which exceed those achieved through the use of solvent mixtures of the prior art.